In early tiers if they see you and hit you, you will die fast, since most tanks don’t have any armor, so solution is to not get exposed, not charge in alone, wait for occasions or make yourself occasions to push but take into consideration that once 1 enemy sees you, they all do( on the minimap) and if they have line of sight on you, you will insta die.

Armor if used properly will help you deflect projectiles, without taking any damage. You have to see your tank’s description, but usually tanks are well armored in front, have some armor on the sides, but usually nothing behind.
Having armor is not enough tho, knowing how to use it matters. If a projectile hits you at a 90 degree angle, it will have maximum penetration force, the one stated in the description. But if it hits you at an angle, that penetration force keeps dropping, making the projectile bounce on your armor.

Short tip: try to take shots in your frontal armor and don’t expose sides/behind; try to angle your tank’s position when taking shots so that the shots hit with an angle, and not directly. This applies when shooting at someone also, try to hit sides/behind at a perpendicular angle, direct hits.

By the way, most tanks have weaker armor even in forward armor, the under side, the one near the ground. So shoot there, and when driving your tanks try to keep your underside covered by a rock, debris, a hill, whatever.

You like SPG…for that i can’t give much advice, it’s kind of self explanatory; you need line of sight to shoot targets, you need to anticipate your shots if the target is moving…what’s good to know is that better players will try counter arty, that means they will try to shoot you by following your projectile’s tracer. That means it’s best to usually move a few meters, while reloading, to avoid getting counter artillery fire.

These are the basics; the rest is just to read up on how vision works in the game, how can you stay hidden, get an ideea about tanks sizes/armor/weak spots in armor etc, good places to be on a map etc. But this you will gain just by playing.